The present invention relates to the field of optical signal processing using photonic structures and in particular to optical signal delay elements.
Periodic dielectric structures have been fabricated which exhibit photonic properties analogous in many respects to the electronic properties of semiconductors. A periodic variation in refractive index can give rise to a photonic band structure in which only certain photonic states are allowed
This is most easily observed in the formation of a photonic band gap Structures exhibiting a photonic band gap forbid the transmission of light in a particular range of frequencies. Structures of this sort are disclosed in WO94/16345 and WO98/53351.
Photonic bandgap (PBG) structures can be formed by a slab of dielectric material having a periodic array of regions having a different refractive index. Holes can be drilled or etched into the material, or an array of columns can be formed Alternatively, stacks of dielectric material of alternating refractive index or a series of slots cut into a dielectric substrate can be used to form a 1-dimensional photonic crystal. The properties of the band structure are determined by the properties of the material and by the geometry of the structure.
Examples of the applications of photonic band structures include the formation of waveguides, use in lasing devices, sensors and even in optical multiplexers and demultiplexers.
According to one aspect of the invention an optical device comprises a delay region having a photonic band structure, an optical input, an optical output, wherein the optical input is adapted to couple an input optical signal of a particular wavelength into a predetermined mode in the delay region such that the optical signal is slowed, and,
wherein the optical output includes a wavelength selective element to select said particular wavelength.
Input optical signals are coupled into a highly dispersive mode in the delay region in which the group velocity of the optical signal is reduced. The input signal, which has been delayed and dispersed, is recovered at the output of the device using the wavelength selective element Input signals comprising components of different wavelength can be used and each wavelength selected at the output.
Without a wavelength selective element the optical output is extremely distorted The highly dispersive nature of the delay region spreads the frequency content of an input optical signal and it is not possible to determine that any part of the signal has been delayed. The processing performed by the wavelength selective element results in the realisation of a delayed output.
The wavelength selective element may select wavelength spatially or temporally. Wavelength selection may be achieved by a wavelength selective element separate from the delay region or by a wavelength selection mechanism in which wavelengths are separated within the delay region. In the latter case, the wavelength selective element in the optical output is a correctly positioned output waveguide. Spatial wavelength selection may be achieved through mechanisms such as filtering, refraction, diffraction and interference. Temporal wavelength selection takes advantage of the fact that different wavelengths undergo a different delay The output signal can be gated to separate different wavelengths.
Preferably, the delay region comprises a first material having a first refractive index including an array of regions having a second refractive index. Preferably, the array extends over a plane in two dimensions. Alternatively, the delay region may be a 1-dimensional photonic crystal formed from a stack of dielectric slabs with alternate slabs forming the array of regions having a second refractive index, or a series of slots cut into a substrate material
The array of regions having a second refractive index gives rise to a photonic band structure. The characteristics of the band structure are dependent on the geometry and material properties of the array of regions The frequency response of the delay region is therefore dependent on the geometry and material properties of the array of regions.
Preferably, the array has a low order of symmetry In particular, the order of rotational symmetry about a point in the array is preferably less than four A lower order of symmetry gives rise to a less uniform band structure, i.e. a more rapid variation of frequency with wave vector. This gives rise to a greater rate of change of group velocity around the band edges.
Preferably, the array of regions includes one or more defects This allows the band structure to be tuned more easily as it gives rise to a high Q-factor for the array The defect could, for example, be a missing region in the array, a displaced region or an enlarged or reduced region within the array. Alternatively, it could be a region within the array having a different refractive index to the rest of the array.
Preferably, the defect is formed from a superposition of two arrays. The superposition of lattices results in a Moire type structure which responds in a similar manner to a set of defects introduced into a single array and is easier to design. Having a set of defects allows light to be coupled into a defect mode more easily than for a single defect. Furthermore, having a large number of defects introduces flat bands in the band structure which allows greater optical delays to be achieved more readily.
Preferably, the first material is silicon nitride or silicon oxynitride.
The delay region may be adapted to allow the transmission of optical signals therethrough, but preferably is adapted to predominantly reflect optical signals of a particular wavelength of operation.
The frequency response of the delay region may be tuned by varying the temperature of the delay region. This causes expansion or contraction of the delay region and hence alters the geometry of the array. Alternatively a piezoelectric material could be used.
Alternatively, the frequency response of the delay region may be tuned by altering the refractive index structure of the delay region. This can be achieved by changing the material composition of the regions, for example when the array of regions is formed from an array of holes in a slab of material, the composition of the material filling the holes can be varied It can also be achieved by forming either the first material or the array of regions from an opto-electric material and applying a potential difference across the delay region.
The direction of incidence of optical signals relative to the array can be altered to obtain a different frequency response from the delay region. Preferably, this is achieved by rotation of the delay region relative to the optical input and optical output.
Preferably, the optical device is adapted to cause optical signals from the input to undergo multiple passes of the delay region. The greater the optical path length within the delay region the greater the delay on the optical signal.
The optical device may be adapted such that an input optical signal undergoes a plurality of passes through a delay region The optical device may also include multiple delay regions Input optical signals would then pass through each delay region in turn at least once.
Preferably, the optical device includes a delay region and waveguides, the waveguides causing multiple passes of input optical signals through the delay region.
More preferably, the optical device includes two delay regions arranged parallel to one another, each adapted to reflect the input optical signals toward the other, such that, in use, input optical signals undergo a plurality of reflections before reaching an optical output. Preferably, waveguides are positioned between the two delay regions to receive the reflected signals. The delay regions may be stacks of dielectric slabs of alternating refractive index arranged parallel to one another.
The wavelength selective device may be a simple optical filter.
The optical device may be adapted so that the delay region diffracts optical signals as well as slowing them. The optical output or outputs can then be placed at particular angular positions to receive particular orders of diffraction. The use of a diffracted beam as an output signal provides automatic wavelength selection. The delay region thus acts as the wavelength selective element. This combined functionality is achieved by matching the effective grating pitch of the delay region to the wavelength of operation whilst also coupling the input signals into a suitable mode.
The optical input may be arranged at an angle to an input or output facet of the delay region such that the input optical signal is refracted Owing to the dispersive nature of the delay region, different wavelengths travel at different speeds within it and hence will refract through different angles Therefore, by positioning the optical output to receive light refracted at a particular angle, wavelength selection is achieved. With the input at an angle to the input facet of the delay region the input optical signal is refracted at the input facet to spatially separate different wavelengths at the output facet. With the input normal to input facet but at an angle to the output facet the signal is refracted at the output to angularly separate different wavelengths at the output facet.
The optical device may form part of a phase-arrayed waveguide grating. The delay region is positioned in an input star coupler or a Multi-Mode Interference (MMI) region whilst the waveguides in a Mach-Zender type arrangement form the wavelength selective element.
Alternatively, the wavelength selective element may be an optical gate adapted to sample an optical output at different times. The sampling rate is dependent on the bit rate of the input optical signal.
According to a second aspect of the present invention, an optical device comprising a delay region having a photonic band structure, an optical input and an optical output,
wherein the optical input is adapted to couple an input optical signal of a particular wavelength into a particular mode in the delay region such that the optical signal is slowed; and,
wherein the delay region is adapted to predominantly reflect the input optical signal at the particular wavelength of operation to allow the input signal to be coupled into a highly dispersive mode.
According to a third aspect of the present invention, an optical device comprising a delay region having a photonic band structure, an optical input and an optical output,
wherein the optical input is adapted to couple input optical signals into a particular mode in the delay region such that the optical signal is slowed; and,
wherein the optical device is adapted to cause the optical signals from the input to undergo a plurality of passes through the delay region to thereby increase the optical path length of optical signals in the delay region.
The optical device may include two delay regions arranged parallel to one another, each adapted to reflect the input optical signals toward the other, such that, in use, input optical signals undergo a plurality of reflections before reaching an optical output.
According to a fourth aspect of the present invention a method of applying a delay to an optical signal comprises the steps of:
coupling the optical signal into a particular mode in a photonic band structure; and,
selecting a part of the optical signal output from the photonic band structure, the selection being made on the basis of wavelength.